What is the large letter at the beginning of each chapter in a book called?

The letters at the beginning which are larger than the others? They're called an Initial. From Latin; initialis, "the start". They are just larger letters, sometimes they put drawing inside them.

This is completely wrong, and was totally unhelpful. I don't know if this is the official "Name" of the large capital letter that begins chapters, but in Word it's called a Dropped Capital. or Dropcap. Word will automatically insert a drop cap into your paragraph if you click the insert tab, and under the text group pick Drop Cap.